This invention relates to an arrangement for joining the edges of lightweight panels, and more particularly to adhesively connected joints with reduced likelihood of peeling failures.
Aircraft and spacecraft structures are required to have relatively high strength in combination with light weight. Such structures may be made from panels, such as panels 10 and 12 of FIG. 1a, joined along one or more edges. For improved stiffness, such a structural panel is often made with a pair of parallel sheets of relatively high-strength, load-bearing material, held in a spaced-apart relationship by a lightweight, low density core. Such panels are often known as "sandwich" panels. The core of such panels holds the load-bearing sheets in a relationship for making the best use of their strength, much as the web of an I-beam holds the principal load-bearing flanges. In FIG. 1a, panel 10 includes first and second "face" sheets or plates 10.sup.1 and 10.sup.2 of aluminum, held together by an aluminum honeycomb 9. Face sheets 10.sup.1 and 10.sup.2 are adhesively bonded to honeycomb core 9. Similarly, a second sandwich panel 12 includes load-bearing aluminum face sheets 12.sup.1 and 12.sup.2, held in a parallel, spaced-apart relationship by honeycomb core 13. FIG. 1b is a cross-section of panel 12 of FIG. 1a, illustrating the honeycomb 13 by stippling. Panels 10 and 12 of FIG. 1 may be joined along their juxtaposed edges 90 and 92.
A variety of panel fastening methods have been used in the past. One of the difficulties associated with joining sandwich panels such as panels 10 and 12 of FIG. 1a is the relative ease with which the low-density core can be crushed, which means that simple techniques using compression fasteners such as bolts may not be reliable. Therefore, bolted structures of sandwich panels may require reinforcement in the region of the bolts, which tends to increase the weight of the structure.
The need to use bolts to fasten sandwich panels is eliminated by the use of adhesive fastening techniques. FIG. 2a illustrates portions of two panels 10 and 12, in which the end or edge 90 of panel 10 is butted against the side of panel 12 adjacent its edge 92. As illustrated in FIG. 2a, skin 12.sup.2 of panel 12 lies adjacent to core 9 of panel 10 in the abutting region. Simple adhesive fastening of the core and face sheet edges of panel 10 to the skin of panel 12 may not result in a high-strength joint, because low density core 9 of panel 10 may not have sufficient strength by itself to adequately transfer loads. Also, such an adhesive fastening method connects only the edges of face sheets 10.sup.1 and 10.sup.2 to the side of face sheet 12.sup.2 of panel 12. The edges of face sheets 10.sup.1 and 10.sup.2 which are adjacent to face sheet 12.sup.2 of panel 12 have such a small area that even a strong adhesive will provide a relatively feeble bond. The prior art arrangement of FIG. 2a improves upon such a simple adhesively connected butt connection by providing reinforcing corners, illustrated as 14 and 16. These corners may be made from the same material as the face sheets, in the example aluminum plate. Inside corner reinforcement 14 is adhesively connected to face sheet 12.sup.2 of panel 12 and to face sheet 10.sup.2 of panel 10 by an adhesive bond illustrated as 20. Similarly, outside corner reinforcement 16 is adhesively bonded to face sheets 12.sup.1 of panel 12 and 10.sup.1 of panel 10 by an adhesive bond illustrated as 22. Outside corner reinforcement 16 is also adhesively bonded to core 13 of panel 12 by an adhesive mass illustrated as 18. As illustrated in FIG. 2a, core 13 is a honeycomb, and therefore the exposed edge of core 13 which faces outside corner reinforcement 16 includes voids, which are filled by adhesive mass 18. As is known to those skilled in the art, inside corner reinforcement 14 and outside corner reinforcement 16 are spaced away from the adjacent stress-carrying sheets or skins of the panels, as by mixing hollow 5 mil (0.005 inch) glass beads with the adhesive prior to its application, to maintain a 5 mil spacing to guarantee that the adhesive is not squeezed from the joint, to maintain the strength of the adhesive. Other spacing methods are well known, as for example, use of a mesh screen or scrim, which also prevents the adhesive from being squeezed from between the structural members being bonded.
In FIG. 2a, forces applied in the direction of arrow 210 to panel 12 are resisted by panel 10, and results in a shear force along the bonded edge. Corner reinforcement 14 has a relatively large bond area 20 with face sheet 12.sup.2, and bears directly against face sheet 10.sup.2. The shear force represented by arrow 210 is resisted by the panel adhesive bond generally, and in particular by inside corner reinforcement 14, with some assistance from outside corner reinforcement 16. Forces tending to pull panel 10 away from panel 12, such as force in the direction of arrow 212 of FIG. 2a, results in a tensile force at the bonded edge. Outside corner reinforcement 16 has a relatively large bond area 22 with face sheet 10.sup.1 of panel 10, and bears directly against face sheet 12.sup.1 of panel 12. The tensile force represented by arrow 212 tends to be resisted by the panel adhesive bond generally, and in particular by outside corner reinforcement 16, with some assistance from inside corner reinforcement 14.
It has been found that when rotational moments, illustrated in FIG. 2b by arrows 36 and 38, are applied to a structure similar to that of FIG. 2a, a "peeling" failure may result. FIG. 2b illustrates such a failure. In FIG. 2b, elements corresponding to those of FIG. 2a are designated by the same reference numerals. In FIG. 2b, the moments represented by arrows 36 and 38 tend to bend outside corner reinforcing element 16 at a point 42, and to bend inside reinforcing element 14 while causing its inside corner to pull away from the adjacent adhesive, leaving a gap or cavity illustrated as 40. Once reinforcing element 14 has peeled away from stress-carrying face sheet 12.sup.2, reinforcing element 14 can not provide as much support against simple shear forces because at least a part of its adhesive bond has failed.
An improved fastening method is desired.